FIG. 4(a) shows a circuit of a prior art microwave switch. In FIG. 4(a), 1 is an RF input terminal and 2 and 3 are a first and a second RF terminal, respectively. A first and a second FET 6, 7, each having a gate terminal 4, are connected between the RF input terminal 1 and the RF output terminals 2, 3, respectively. Inductances 5 are connected in parallel with the FET 6 and the FET 7.
FIG. 4(b) shows an equivalent circuit of an FET when the FET is off and FIG. 4(c) shows an equivalent circuit when the FET is on.
This switch operates as follows. In order to connect an RF signal input from the input terminal 1 and to the first output terminal 2, a zero volt signal is applied to the gate terminal 4 of the FET 6 to turn off the FET 6. A voltage not exceeding the FET pinch-off voltage, for example, -5 V, is applied to the gate terminal 4 of the FET 7 to turn off the FET 7. Then, the FET 6 and the FET 7 are equivalent to the circuits of FIGS. 4(c) and 4(b) respectively. Since only a resistance appears between the terminals 1 and 2, the RF signal is transmitted from the terminal 1 to the terminal 2. On the other hand, a resonance circuit comprising the inductance 5 connected with the FET 7 and the capacitance of the FET 7 resonates between terminals 1 and 3 at a desired frequency, and the electrical state of the FET 7 appears as an infinite impedance. Therefore the RF signal is not transmitted from the terminal 1 to the terminal 3. In addition, when the voltages applied to the gate terminals 4 of FETs 6 and 7 are reversed, the RF signal is transmitted to the second output terminal 3.
FIG. 5 shows a prior art microwave switched-line type phase shifter. In FIG. 5, the same reference numerals denote the same elements as those shown in FIG. 4. Reference numerals 8 and 9 designate a third and a fourth FET, respectively. 10 and 11 are a first and a second transmission line, respectively.
This phase shifter operates as follows. When the FET 6 and the FET 7 are turned on and the FET 8 and the FET 9 are turned off by controlling the voltages applied to the gate terminals 4, the RF signal which is input from the input terminal 1 is output to the output terminal 2 through the first transmission line 10. On the contrary, when the FET 6 and the FET 7 are turned off, and the FET 8 and the FET 9 are turned on, the RF signal input from the input terminal 1 transmitted through the second transmission line 11. In these cases, since the transmission lines 10 and 11 have a predetermined difference in electrical length, the phase shift between the two on states can be varied.
FIG. 6 shows a prior art loaded line type phase shifter. In FIG. 6, the same reference numerals denote the same elements as those shown in FIGS. 4 and 5 and 12 is a third transmission line. This loaded line type phase shifter includes a one-quarter wavelength transmission line 12 and two series connection transmission lines and FETs. The parameters of the series connected pair of transmission lines and FETs (a pair 10 and 6, and another pair 11 and 7) are chosen so that the susceptance viewed from the main line 12 is =jB when the FETs 6 and 7 are on and +jB when the FETs 6 and 7 are off. Then, the susceptance value B is related to the phase shift by, EQU B=tan (A/2)
A=the phase shift Then, the length of the main line 12 is chosen to produce a predetermined difference between the phase of the input signal and that of the output signal of the main line 12.
In the above-described impedance control circuit used as a microwave switch, a switched-line type phase shifter, or a loaded-line type phase shifter, the circuit is controlled through the capacitance of an FET in its off state. However, this capacitance value of an FET varies to a large extent due to variations in the FET fabrication process, thereby varying the circuit characteristics to a great extent. FIGS. 7(b) and 7(c) show the transmission characteristics of the resonance circuit of FIG. 7(a) when the capacitance of the FET is 0.15 pF and 0.1 pF, respectively, with the inductance 5 being 2 nH. As is apparent from these figures, when the capacitance of the FET 5 when in the off state is 0.15 pF, the attenuation amount S.sub.21 of the output terminal 2 viewed from the input terminal 1 is at a maximum at about 9 GHz, and when it is 0.15 pF, the attenuation amount S.sub.21 is at a maximum at about 11 GHz. Thus, the resonance frequency at which the impedance becomes infinite varies to a great extent due to the variations in the characteristics of the FET 5.